This invention relates to an acoustic flowmeter and more particularly to such a flowmeter having improved accuracy and reliability.
Various systems have been developed for measuring the velocity of a fluid flow by utilizing the effect the fluid velocity will have on the transit time of acoustic pulses transmitted through the fluid medium along the axis of flow. One such system uses a dual "sing-around" technique in which one series of pulses is transmitted in one direction, e.g. upstream, and another series of pulses is transmitted in the opposite direction, e.g. downstream. The time when each pulse is received determines when the next pulse will be transmitted in the same direction. Accordingly, the repetition rate of the pulses in each direction is predominantly determined by the transit time between spaced transducers in the fluid medium. The transit time, in turn, is proportional to the flow velocity of the fluid medium. Thus, the pulse repetition rate provides an indication of fluid velocity.
Other systems have attempted to measure directly the elapsed time between transmission and reception of acoustic pulses in the form of repetitive triggered bursts of high frequency acoustic energy having a characteristic amplitude envelope. Because the beginning and end of the bursts is low in amplitude, one approach is to detect the peak amplitude of the envelope signal, and to measure time from peak to peak. However, because of the signal to noise ratio of broadband transducers at the relatively low acoustic frequencies employed, e.g. 500 kHz and below, precise peak detection has proven to be difficult. It would be desirable to have a system of measuring the time between transmission and reception of the signal envelope that is more accurate.
Accordingly, it is an object of this invention to provide an acoustic flowmeter that includes accurate, reliable, and inexpensive acoustic pulse transit time measuring system which does not rely on direct peak detection.